Utility system electric loads vary from hour to hour, day to day, and from season to season. These load variations result in typical annual electric utility system load factors ranging from about 50 to 70 percent. Utility system generating capacity is installed to meet the system's annual peak daily load requirements with a certain degree of reliability. Therefore, the amount of total installed generating capacity is generally in the range of 15 to 25 percent higher than the system's peak day load demand. Because of the difference between the variations in the system load profile and the generation capacity reserve margin at any time, a portion of a system's baseload installed generating capacity may be unused during off-peak or light load periods. This availability of off-peak base-load generating capacity makes electric energy storage concepts attractive to utilities because of the potential savings which could result from storing low cost base-load off-peak energy for use in supplying peak load requirements. The benefits of electrochemical energy storage devices are expected to be realized more fully in conjunction with nuclear power plants which are designed as base load plants for optimal economy of operation rather than with base-load fossil fuel plants whose fuel supply is limited, unstable and increasing in cost as lower sulfur content fuel is needed to meet air pollution control requirements.
Off-peak energy storage requirements have been met in the past by the use of hydroelectric pumped water storage on electric utility systems. Lack of acceptable sites for future pumped storage systems has increased the desirability of developing alternative off-peak energy storage systems. Electrochemical energy storage systems are potentially attractive for future use in electric utility systems. The two general types of electrochemical systems being considered are: rechargeable chemical batteries (e.g., lithium sulfur and lead-acid batteries) and hydrogen systems. One type of hydrogen system is based on a water electrolysis subsystem to generate hydrogen, a metal hydride subsystem to store hydrogen, and a hydrogen/air (oxygen) fuel cell subsystem. The electrolyzer/fuel cell combination (two separate devices) with hydrogen, and possibly oxygen, storage constitutes an off-peak energy storage system. This use of a fuel cell is distinguished from a reformed-type fuel cell operating on fossil fuels as a source of hydrogen to generate peaking power.
The water battery or reversible electrolyzer of the present invention is distinguished from the above hydrogen system in that the same hardware or device is used to perform both the water electrolysis and the fuel cell functions within a closed system recycling the water and gases between the device and their storage facilities. The general concept of using a single device for off-peak energy storage is not new. The term "regenerative (hydrogen-oxygen) fuel cell" has been used to describe several different concepts that were variations of fuel cell technology for space applications: a single electrolyzer/fuel cell device with internal gas storage, separate electrolyzer and fuel cell devices in a common unit with internal gas storage, separate water electrolysis and fuel cell subsystems with external gas storage.
The water battery is a name we are using for a water electrolyzer capable of being operated in the reverse (fuel cell) mode to generate electrical energy. The name distinguishes this reversible water electrolyzer concept from other water electrolysis, fuel cell, or regenerative fuel cell concepts being considered by others as an energy storage system.
Large-scale water electrolysis units that have been developed over the past 50 years for use in industry (primarily foreign countries rather than the U.S.) are not capable of being operated in the reverse or fuel-cell mode. Present industrial electrolyzers designed for low temperature operation (less than 100 C) for long life are relatively inefficient. If used in conjunction with advanced fuel cells, the round-trip efficiency for an energy storage system would be significantly less than 50 percent. In addition, a separate water electrolysis unit and a separate fuel cell would occupy about twice the space and involve nearly twice the capital investment. Each unit would be idle a significant portion of the daily cycle in an energy storage system using off-peak energy.
Regenerative hydrogen-oxygen fuel cells as the name implies were outgrowths of fuel-cell development that were subsequently investigated for conventional battery applications such as for use in outer space. However, use of electrodes designed for fuel-cell operation as electrodes for water electrolysis severely limits the electrode life or the efficiency attainable if used at low temperatures. In contrast to regenerative fuel-cells, the term reversible water electrolyzer is meant to imply a system designed for water electrolysis with long life and high efficiency that is also capable of being operated in the reverse mode as a hydrogen-oxygen fuel cell.
Rechargeable chemical batteries are distinguished from fuel cell systems because the active materials are retained within a closed system rather than being supplied continuously from an external source. It is common practice to name the battery system for the active materials or reactants, e.g., Ni-Cd (nickelic hydrated oxide-cadmium), Ag--Zn (argentic oxide-zinc) batteries. The elemental name of the chemical battery reflects the active materials which are usually economic considerations (cost, availability) of concern for large energy storage capacity. The active materials in a reversible water electrolyzer are water, hydrogen, and oxygen.
There are alternative modes of storing the active materials in the charged water battery (e.g., hydrogen as a gas or as a metal hydride). However, in the discharged battery, the active material will be stored as liquid water. Thus, the term water battery is appropriate.
In contrast to an energy storage system based on separate water electrolyzers and H.sub.2 /O.sub.2 fuel cells, the water battery operates similarly to chemical batteries in that the same electrodes are used for both charge and discharge. A water battery, or reversible electrolyzer, will operate alternately in both the charging and discharging modes. During charging, electric energy is used to electrolyze water, and the oxygen and hydrogen gases generated are stored externally to the cells. During discharging the stored hydrogen and oxygen are reacted together electrochemically to provide electrical energy and the water produced is stored externally to the cells. Both the positive and the negative electrodes must be capable of reverse operation. Of the two modes of operation, the charging mode is more demanding of materials requirements because of the oxidizing conditions at the oxygen electrode.
The electrolysis cell which is the basis of the water battery concept was developed over a period of several years at Battelle's Columbus Laboratories. The cell is characterized as a long-life water electrolysis cell for operation in the intermediate temperature range (150.degree. C to 250.degree. C) at high efficiency. Laboratory water electrolysis cell units have been operated at over 200.degree. C and various current density levels for extended periods of time (up to 120 days) and have shown stable voltage behavior (no performance deterioration with time). Electrodes have been operated in the water electrolysis mode at 145.degree. C for over 16,000 hours with no performance deterioration.